The present invention relates to a cooling system having a common cooling circuit for cooling the internal combustion engine, on the one hand, and the charge air and possibly other media, on the other hand; wherein the discharge of the coolant from the engine is controlled by means of a thermostat, and the coolant leaving the engine is then returned successively through a so-called skin cooler (which is operated on raw water) an intercooler (aftercooler), and possibly other coolers back into the cooling jacket of the engine. A cooling system of this general type is disclosed in German Preliminary Pat. No. 1 476 384.
As a rule, water-cooled marine Diesel engines are cooled by means of fresh water, which is circulated in a closed circuit through the cooling jackets of the engine by means of a water pump driven by the internal combustion engine, and which is subsequently subjected to a heat exchange process. The heat exchange process takes place in a cooler (shell-and-tube water-to-water cooler) which is operated on raw or untreated water (e.g. fresh or salt water). In view of increasing water pollution, river and coastal vessels increasingly resort to skin-cooling systems. In these systems, the engine jacket water is cooled in a skin cooler which is fitted in the hull and which dissipates the heat to the water through which the vessel moves.
The use of untreated water for cooling is attractive in the case of marine engines, if only because the cooling water is available at a low temperature level (the temperature of the river or sea), which is important in view of the desired and necessary cooling of the charge air for a high engine output.
Where a common cooling circuit is adopted for cooling the internal combustion engine, on the one hand, and the charge air and possibly other media (e.g. the engine and gear oil), on the other hand, it is necessary to maintain the engine or combustion temperature at a higher level than that of the charge air and/or other media. The temperatures for the jacket coolant are roughly between 78.degree. and 85.degree.C., whereas the temperature of the charge air has to be maintained at a level below or equal to 45.degree. C.
Common cooling circuits which solve this problem have been disclosed.
For instance, the published European patent application No. 54792 shows a cooling circuit where, in order to ensure a good heat transfer rate, the full engine coolant flow is passed through an air/water main cooler to be then subdivided into two partial flows. One of these flows is routed through a cooler which is constructed as a secondary cooler and an intercooler or charge air cooler for the charge air circuit, and then is united again with the other partial flow (the main or jacket water circuit) before entering the internal combustion engine. The split-up is effected either by an "expensive" partial flow control valve, which is controlled as a function of temperature or time, or by a (simplified) flow control valve (variable-area orifice) that is provided in the secondary circuit and is controlled by the system pressure or temperature.
A drawback of this arrangement is the fact that, in addition to the intercooler, another cooler (i.e. a secondary cooler) is required, and that another partial flow control valve, which is dimensioned for the complete circulating water flow and is located upstream of the air/water main cooler, is needed for the warming-up phase of the internal combustion engine. Moveover, the proposed splitting of the flow by means of the second partial flow or flow control valve is extremely problematic, especially in the case of engines with a wide operating speed range and with the commonly used constant drive ratio of the water pump driven by the engine, since the flow pressure varies widely as a function of the water pump speed. Due to the control inertia, dangerous operating conditions are liable to occur in the intercooler under conditions of drastic load changes.
Another prior disclosure provides for a liquid circuit of a supercharged internal combustion engine with positive circulation of the coolant successively through the engine, radiator, and intercooler, and for a bypass around both coolers, which is characterized by the coolant flow leaving the engine and being split into two partial flows, each of which is controlled by a control valve. One control valve is controlled as a function of the charge air temperature in the intercooler and the other control valve is controlled as a function of the temperature of the coolant entering the engine. Each control valve is formed with two outlet channels, of which one outlet channel joins the bypass line around the coolers and one outlet channel joins the line leading to the water cooler (German Preliminary Pat. No. 1 476 384).
A drawback of this cooling system, which incidentally uses a skin cooler, is the fact that two control valves are used, which are controlled as indicated in the foregoing. As a result of this, this system fails to achieve the quick warming-up required for high-performance engines after start-up (i.e. it is impossible to attain the correct operating temperature within a few minutes). This is due to the fact that since, when the charge air is hot, a portion of the cooling water is withdrawn from the engine circuit and is then intensively cooled in the main cooler (radiator) to be subsequently heated again, but only slightly, in the intercooler and the oil cooler.
It is therefore an object of the present invention to provide a cooling circuit of the aforementioned general type defined above, and of the simplest construction possible. It is a further object of the invention to provide rapid (i.e. within a few minutes) warming-up after cold starting for high-performance engines where the engine loading varies within extremely wide limits, and to ensure that maintaining the temperature of the engine at partial load is not affected by the intercooler or charge air cooler. It is also an object of the present invention to maintain intensive cooling of the charge air, since the charge air cooler is capable immediately (i.e. without any time lag) of handling the full cooling load, such as is required under conditions of frequent abrupt load changes.